Mobile Ammonites Stayed Put at Plains Methane Seeps

Research led by Museum scientists shows that ammonites, an extinct type of shelled mollusk that’s closely related to modern-day nautiluses and squids, made homes in the unique environments surrounding methane seeps in the seaway that once covered America’s Great Plains. The findings, recently published in the journal Geology, provide new insights into the mode of life and habitat of these ancient animals.

In the Black Hills region of South Dakota, researchers are investigating a 74-million-year-old mound of fossilized material where methane-rich fluids once migrated through the sediments onto the sea floor. When the face of this cliff recently slumped off, a wide variety of bivalves, sponges, corals, fish, crinoids, and, as recently documented, ammonites, were revealed.

Studying these well-preserved shells, the researchers tried to determine the role of ammonites in the unique seep ecosystem. By analyzing the abundance of isotopes (alternative forms) of carbon, oxygen, and strontium, the group made a surprising discovery. The ammonites at the seep, once thought to be transient, had spent their whole lives there. The seeps also likely attracted large clusters of plankton, the ammonites’ preferred prey.

“Ammonites are generally considered mobile animals, freely coming and going,” said Neil Landman, lead author of the Geology paper and a curator in the Division of Paleontology at the Museum. “That’s a characteristic that really distinguishes them from other mollusks that sit on the sea floor. But to my astonishment, our analysis showed that these ammonites, while mobile, seemed to have lived their whole life at a seep, forming an integral part of an interwoven community.”

With these findings in mind, the researchers think that the methane seeps probably played a role in the evolution of ammonites and other faunal elements in the Western Interior Seaway. The seeps might have formed small mounds that rose above the oxygen-poor sea floor, creating mini oases in a less-hospitable setting. This could be a reason why ammonites were able to inhabit the seaway over millions of years in spite of occasional environmental disturbances.

An overview of the outcrop where the ammonites were found. Photo courtesy of Larry D. Stetler